1. Field of the Invention.
This invention relates to an output buffer wherein the drive strength of the output buffer is programmable. More particularly, this invention relates to an output buffer connected to an ISA bus, wherein the drive strength is programmable for a supply voltage range of 3.3-5.0 volts with a capacitive load range from 60 pF to 240 pF.
2. Brief Description of the Related Technology
Input/output (I/O) buffers for ISA buses currently are being designed to operate in 5.0 volt and 3.3 volt systems. Pulldown circuits have been used in 5.0 volt systems to improve system performance. Examples of I/O buffers designed for 5.0 volt systems having a pulldown circuit are as follows: U.S. Pat. No. 5,248,906, entitled, High Speed CMOS Output Buffer Circuit Minimizes Output Signal Oscillation and Steady Stay Current, and U.S. Pat. No. 5,321,319, entitled, High Speed CMOS Bus Driver Circuit That Provides Minimum Output Signal Oscillation, both by Mahmood and both assigned to the common assignee of the present invention. These references describe a slew rate controlled output buffer circuit that transitions a pulldown output transistor through several transistor levels (VTNs), to prevent current transient noise during the pulldown process for VCC=5.0 volts.
A need exists for a I/O buffer output buffer driver capable of operating in either a 5.0 or 3.3 volt environment. For 3.3 volt operation, the 5.0 volt pulldown transition circuitry of the prior art is unnecessary since current transients are inherently minimized for 3.3 volt systems.
A need also exists for an I/O buffer capable of driving capacitive loads on an ISA bus ranging from 60 pF to 240 pF, at 5.0 volts and 3.3 volts. The two prior art references discussed above do not provide this capability.